I. Field of the Invention
The present invention relates to radio communications. More particularly, the present invention relates to cellular radiotelephone communications.
II. Description of the Related Art
Cellular radiotelephone systems enable mobile subscribers to communicate with land-line telephone networks while moving through a geographical area. High density, high capacity cells in typical cellular radiotelephone systems are made up of six directional antennas centrally located in the cell. Each antenna radiates into a 60.degree. sector of the cell. A number of these cells are combined to form the cellular radiotelephone system.
The cell shapes are determined by both the radiation pattern of the antennas and the local conditions at the cell site. Cells, however, are typically idealized as hexagonal patterns since such a pattern closely approximates the ideal antenna radiation pattern.
Cellular radiotelephone systems use different channel frequencies for each mobile subscriber. The transmission from the mobile to the cell uses one frequency while the transmission from the cell to the mobile user uses another frequency. These two frequencies are not used by other nearby mobiles as this would lead to interference on the channel and a reduction in signal quality. This interference is referred to in the art as co-channel interference.
Another type of interference experienced by mobile subscribers is adjacent channel interference. This interference is due to the energy slipover between adjacent channels.
Both types of interference affect the signal quality, referred to as the carrier to interference ratio (C/I). This ratio is the signal strength of the received desired carrier to the signal strength of the received interfering carriers. A number of physical factors can also affect C/I in cellular systems including: buildings, geography, antenna radiation patterns, mobile traffic transmitting power, and mobile traffic location within the cell.
Due to the low power of the cell's transmitters, the same frequencies can be reused in other cells, referred to as co-channel cells, in the same geographical area. Greater frequency reuse allows more mobile traffic to use the cellular system. There are, however, constraints on the location of the co-channel cells. Even though the transmitters are typically low power, placing co-channel cells too close may cause interference.
Frequency planning optimizes spectrum usage, enhances channel capacity and reduces interference. A frequency plan also ensures adequate channel isolation to avoid energy slipover between channels, so that adjacent channel interference is reduced. Moreover, an adequate repeat distance is provided to an extent where co-channel interference is acceptable while maintaining a high channel capacity. In order to accomplish these diverse requirements, a compromise is generally made so that the target C/I performance is acquired without jeopardizing the system capacity. However, the existing frequency planning schemes do not always permit this. As a result, with growing cellular subscribers, today's cellular networks are overloaded and do not provide an adequate service.
A prior art method of symmetrical frequency planning begins with two integers, i and j, that are referred to as shift parameters. The frequency plan is established by starting with a reference cell and moving over i cells along the chain of cells. After reaching the i.sup.th cell, a counter-clockwise turn of 60.degree. is made and another move of j cells is made. The j.sup.th cell can safely be a co-channel cell. The frequency plan can also be established by moving j cells before turning i cells or by turning 60.degree. clockwise.
After all the possible co-channel cells of the initial cell are laid out, another reference cell is chosen and the procedure is repeated. this entire procedure is repeated as often as necessary to establish the frequency plan of the entire metropolitan cellular system.
The cells thus established by the above procedure form a reuse pattern of i.sup.2 +ij+j.sup.2 cells. The number of cells in this reuse pattern is a predominant concern of the cellular industry since this number determines how many different channel groups can be formed out of the frequency spectrum allocated to cellular radiotelephones. A low number of cells in a reuse pattern means more channel groups can be formed and more users accommodated.
Another method for determining frequency reuse is illustrated in FIGS. 1 and 2. FIG. 1 illustrates a conventional N=4, 60.degree. sectorized cell cluster. This system is based on dividing the available channels into 24 frequency groups that are distributed evenly among four 60.degree. cell sites (a cluster). One frequency group is allocated per sector.
FIG. 2 illustrates a growth plan using the cluster of FIG. 1. Each cluster is repeated throughout the network. As a result, there are at least five co-channel sites that are randomly oriented. The C/I performance, therefore, is due to the five randomly oriented co-channel interferers.
The reuse distance of this plan is D/R=.sqroot.4.times.3=3.46. From the reuse distance, the C/I prediction equation, well known in the art, is estimated as: ##EQU1## where .gamma. is the pathloss slope and .DELTA.dB is due to the antenna directivity. The antenna directivity is typically 3-5 dB, depending on the antenna beam width.
The pathloss slope, also referred to in the art as the propagation constant, is well known in the art. This constant is the rate of decay of signal strength as a function of distance. The pathloss slope increases as the environment surrounding the cells becomes more crowded with buildings and other obstructions. These values range from 2 for free space with no obstructions, 2.5 for rural areas, 3-3.5 for suburban areas, 3.5-4 for urban areas, and 4-4.5 for dense urban areas.
In the above example, .DELTA.dB is conservatively estimated at 5 dB. In this case, C/I.apprxeq.19.6 dB. Additional measures, such as antenna downtilt and beam width, have to be properly engineered for satisfactory performance and operation of this system.
In the Advanced Mobile Phone System (AMPS) used in North America, 416 frequencies are available for cellular use. These 416 frequencies must be divided among the cells and reused as necessary to provide coverage to a metropolitan or other geographical area. Additionally, 21 of these channel frequencies must be used as control frequencies to establish calls between users and cells. This further reduces the number of channels to 395 for use as voice channels.
In the above N=4 system, the number of voice channels per sector is 395/24.apprxeq.16. This results in 16.times.6=96 voice channels per cell.
There is a need for even greater frequency reuse to expand the capacity of the cellular system without performance degradation. There is an additional need for an economical method of frequency reuse.